Electrical power generating systems, such as electromechanical actuators and integrated drive generators (IDGs) employed in military and commercial aircraft, often utilize power electronic components to meet stringent design constraints and high power requirements. These electronic devices can include components secured to printed circuit boards by surface mount technology (SMT) as well as plated through-hole (PTH) components. Such components are typically soldered to a printed circuit board by either a wave soldering or reflowing soldering process. Substantial care must be taken during each of these manufacturing processes because the individual components tend to be brittle and the electrically conductive leads which attach the components to the board are fragile.
A series of manufactured printed wiring boards can be interconnected together to form a modular unit, such as an electronic control unit (ECU) for use with electric power generating systems. The ECU circuitry provides the conductive framework through which electrical signals are transmitted in order to provide power to various systems of the aircraft. The electronic components mounted on that circuitry can transmit such electrical signals through the circuitry.
Before the electronic components are mounted and soldered to the printed wiring board, it is a common practice to apply a solder mask to the circuitry. The solder mask acts to protect the circuitry from corrosive inhibitants. One solder mask available from Hysol Corporation is a dry film polymer solder mask which cures on the printed wiring board at a high temperature in an oven. However, as a dry film solder mask is susceptible to heat, care is required to minimize any variability in curing to prevent the solder mask from being cured on one portion of the board, but not another.
Once the electronic components are soldered to the printed circuit board, a protective film is typically applied over the entire board surface. Such a protective film is commonly referred to in the industry as a conformal coating. Conformal coatings are usually systems of synthetic resins, such as polyurethane, which when cured can provide a measure of protection of the components against environmental stresses and foreign object damage (FOD). These stresses include chemical corrosion, fungus growth, salt, dust, and fuel contamination, along with physical breakage from mechanical fatigue and shock. Conformal coatings may also serve in electrically insulating the electronic components and circuitry; thus, while they can protect against FOD, they can simultaneously lock any extraneous elements on the board underneath the coating. U.S. Pat. No. 5,246,730 to Peirce et al. and U.S. Pat. No. 4,830,922 to Sparrowhawk et al. are illustrative of such conformal coatings and discuss preferred minimum thicknesses of coatings which are applied to printed wiring boards.
In addition, printed circuit boards which are used in military marine, space and aircraft may be subjected to adverse environmental conditions of humidity and thermal shock. Condensation, which can form under humid conditions, can corrode the electrical components and lower the resistance between conductive elements of the circuit. This can accelerate high voltage breakdowns and eventually cause electrical shorting between elements of the circuit. A conformal coating can protect against condensation in typical situations.
Also, in order to compensate for these adverse conditions, a printed wiring board must be designed to comply with military specifications, such as MIL-C-28809. MIL-C-28809 specifies, among other things, that a manufactured board should pass a thermal shock test. This test entails heating the coated board to an extreme temperature, immediately subjecting the board to subzero temperatures, and then, cycling through these parameters one hundred (100) times.
One problem which has arisen in meeting this thermal shock requirement involves blistering or vesication of a portion of the polyurethane conformal coating. This blistering or vesication can occur either on the surface of an electronic component or from the board surface surrounding the component. Polyurethane conformal coatings are available as either single-part or multi-part curing systems and can provide excellent chemical resistance in certain environments. However, with such resistant properties, rework of a damaged portion of the polyurethane conformal coating becomes extremely difficult, involving labor intensive hours to avoid damaging the components. Thus, such repairs can be costly.
MIL-C-28809 further specifies that a manufactured printed circuit board must pass high humidity testing after being subjected to thermal shock. However, an additional problem can develop in conjunction with high humidity testing when using a polyurethane conformal coating. This problem is referred to as "mealing" or "white residue" and is visible on the surface of the printed circuit board as small, tightly spaced bubbles. Since the "mealing" phenomena tends to proliferate over the entire board surface, it is difficult to remove the complete coating and rework the entire surface, especially without damaging the solder mask or the electronic components' leads. Thus, complete batches of manufactured printed circuit boards can end up being scrapped due to this condition. This excess scrap can result in costly manufacturing operations.
It is, therefore, a primary object of the present inventor to determine the cause of the mealing phenomena, and to invent an improved manufacturing process for preventing such a phenomena from occurring with polyurethane conformal coatings which are applied to printed circuit boards. Other objects of the invention include providing the following:
(i) an improved manufacturing process for preventing vesication of polyurethane conformal coatings applied to printed circuit boards; PA1 (ii) a cost-effective manufacturing process; PA1 (iii) a manufacturing process which substantially eliminates rework of polyurethane conformal coatings applied to printed wiring boards; PA1 (iv) an improved manufactured printed wiring board which can withstand thermal shock conditions; PA1 (v) an improved manufactured printed wiring board which can withstand high humidity conditions; and PA1 (vi) an improved printed circuit board which meets the requirements of MIL-C-28809.